ERα dimerization: a key factor for the weak estrogenic activity of an ERα modulator unable to compete with estradiol in binding assays

Abstract

Estrothiazine (ESTZ) is a weak estrogen sharing structural similarities with coumestrol. ESTZ failed to compete with [³H]17β-estradiol ([³H]17β-E₂) for binding to the estrogen receptor α (ERα), questioning its ability to interact with the receptor. However, detection by atomic force spectroscopy (AFS) of an ESTZ-induced ERα dimerization has eliminated any remaining doubts. The effect of the compound on the proliferation of ERα-positive and negative breast cancer cells confirmed the requirement of the receptor. The efficiency of ESTZ in MCF-7 cells was weak without any potency to modify the proliferation profile of estradiol and coumestrol. Growth enhancement was associated with a proteasomal degradation of ERα without substantial recruitment of LxxLL coactivators. This may be related to an unusual delay between the acquisition by the receptor of an ERE-binding capacity and the subsequent estrogen-dependent transcription. A complementary ability to enhance TPA-induced AP-1 transcription was observed, even at concentrations insufficient to activate the ERα, suggesting a partly independent mechanism. ESTZ also rapidly and transiently activated ERK1/2 likely through membrane estrogenic pathways provoking a reorganization of the actin network. Finally, the systematic absence of biological responses with an ESTZ derivative unable to induce ERα dimerization stresses the importance of this step in the action of the compound, as reported for conventional estrogens. In view of the existence of many other ERα modulators (endocrine disruptors such as, for example, pesticides, environmental contaminants or phytoestrogens) with extremely weak or similar apparent lack of binding ability, our work may appear as pilot investigation for assessing their mechanism of action.

Keywords:

• Dimerization
• ERK1/2
• estrogen receptor
• proliferation
• transcription
• weak estrogen

Acknowledgements

G. Leclercq is Honorary Professor of ULB and G. Laurent is Senior Research Associate of the NFSR. We are grateful to Pr. A.E. Lykkesfeldt (Breast Cancer Group, Danish Cancer Society Research Center, Copenhagen, Denmark) for preliminary experiments of the effects of ESTZ on tamoxifen-resistant MCF-7/TAM^R-1 cells. We also thank Louise Hodler (UMR 7203, University Pierre et Marie Curie) and Dr. D. Gallo (J.C. Heuson Laboratory) for experimental assistance.

Disclosure statement

The authors have no conflict of interest to declare.

Funding

This work was supported by La Ligue National contre le Cancer (comité de Montbéliard), the Center National pour la Recherche Scientifique (CNRS, France) and the National Fund for Scientific Research (NFSR, Belgium).